The invention relates to shaped beam antennas and in particular, to an antenna for forming a sector beam, the shape and position of which are constant over a wide frequency bandwidth.
Shaped beam antennas are useful for many purposes, one of which is efficient energy management. These antennas are becoming more useful in other areas as the sophistication of radar systems increases. The capabilities of precision direction finding and resolution of complex targets where only limited scanning time is available are becoming more and more necessary in view of the speeds and radar capabilities of modern threats. For example, one defense against a radar equipped threat is to steer it off target. However, the effectiveness of signals transmitted to steer the threat off target can depend upon multipath propagation effects which alter the transmitted beam. Multipath propagation can cause fluctuation of 10 to 20 dB, which may distort the antenna beam to a point where it becomes ineffective. The multipath effect becomes a significant consideration in relation to threats which fly low to the ground or water, or close to other stationary clutter.
Multipath fluctuations can be reduced significantly by employing shaped beam antennas which provide steep beam slope or cut off characteristics near the clutter position. For example, where the antenna is located on a ship, a steep beam slope in the elevation plane near the water would be desirable in relation to low flying threats.
In addition, since the frequency or frequencies of the radar system of the threat are typically unknown, a wide frequency bandwidth in the shaped beam antenna is also desirable. Coupling a wide frequency bandwidth with a constantly shaped beam where the beam position and shape are independent of frequency over the wide frequency bandwidth would result in an antenna well adapted for use in high multipath environments.
The principles of geometric optics have been the design basis for prior shaped beam antennas. One prior optical technique involves the offset feed parabolic antenna. A point source whose radiation pattern is known illuminates a reflector shaped such that the feed energy is redistributed into the desired far field shape. These reflectors may also be shaped to provide a narrow beam in the orthogonal plane. This technique is described in more detail in Silver, Microwave Antenna Theory and Design, McGraw-Hill, NY, 1949, pg. 497 et seq. The advantage of this technique is simplicity and low cost. However, the feed pattern is a function of the product of the wave number k=(2.pi./.lambda.) and the sine of the angle .theta., so the beam position and shape in the far field vary with frequency as k sin .theta..
In order to retain a constant far field pattern, a constant feed pattern would be required and this is difficult to obtain. If a nonfocal feed were used, for example an array on a spherical cap, the feed would be broadband, however points on the feed generally would not produce focused pencil beams in the far field, and sharp beam edges would not be obtained. A prior optical technique is radiating directly from a spherical surface. This produces constant beamwidth and constant beam shape over a wide frequency bandwidth, however, again the sharp beam edges have not been obtained.
The above described techniques are based on providing a single beam and shaping that beam as required. Another prior technique involves a constrained approach such as using a corporate or series type transmission line or a waveguide power divider to feed a planar or linear aperture, such as in the Butler beam forming array. Any realizable far field pattern can be produced using this technique and if the Woodward synthesis is used, the aperture size will be small and the edge shape will be the steepest possible corresponding to the aperture size. However, in this approach, the aperture distribution is constant with frequency, therefore the far field pattern shape will vary as k sin .theta.. Also in this technique, operating over too wide a frequency band changes the beamwidth, shifts the location of the beams and can introduce grating lobes just as with any array antenna. In particular, the beamwidth will typically be broadened by a factor of two over a frequency bandwidth of an octave.
A variant of the above described constrained approach is given in U.S. Pat. No. 4,146,896 to Wild (1979), where a spherical cap feeds a planar array. As discussed above, the use of a planar array causes a narrow frequency bandwidth for the structure. Also the beams are not fixed in position and will vary with frequency. In order to obtain a steep beam slope plus a one-half octave or greater frequency bandwidth, the antenna would need an aperture of several hundred wavelengths, which is an impracticable structure in most cases.